(a) Technical Field
The present disclosure relates to a vehicle audio system and a noise removal method thereof, and more particularly, to a vehicle audio system and a noise removal method thereof, which can efficiently remove noise that may be generated in a vehicle audio system employing a full digital amplifier.
(b) Background Art
In general, a vehicle audio system enables listeners to listen to the radio, as well as various sound sources stored in analog form using an external media device (e.g., CD, MP3, etc.). An ordinary vehicle audio system typically uses an amplifier for amplifying audio signals and a speaker for outputting audio signals to provide sounds to the driver and passengers. Meanwhile, a digital amplifier generally refers to an amplifier that directly receives digital signals, amplifies the received digital signals in a digital form without converting the digital signals into analog form, and applies the amplified digital signals to a speaker.
Currently, the process of converting the form of an audio amplifier stage into digital form is unsatisfactory, and therefore, the conventional analog scheme is frequently used. To this point, a conventional audio amplifier used in a vehicle, for example, is configured to include a digital-analog converter and an AB-class amplifier. The conventional audio amplifier receives a pulse code modulation (PCM) signal, modulates the received PCM signal into an analog signal through the digital-analog converter, and amplifies the modulated analog signal through the AB-class amplifier. Also, the amplified signal has noise removed by passing through an LC low pass filter and is then output through a speaker.
However, the conventional amplifier generates a large amount of heat in an amplifier circuit, and accordingly, the size of a heat sink is also relatively large. As the size of the heat sink increases, the design of the vehicle becomes increasingly limited, and the fuel efficiency of the vehicle is also decreased. In addition, a complicated circuit may be necessary to remove noise.
In order to solve the above-described problems of the conventional audio amplifier, a vehicle audio system using a full digital amplifier has recently been made, and the present applicant has filed a patent application [No. 10-2013-016525 (2013-12-27)] related to an audio amplification method and an audio amplification circuit. The method and circuit disclosed therein can solve the problem of excessive heat generation using a full digital amplifier as the vehicle amplifier and can amplify a PCM signal directly input without using a digital signal processing (DSP) chip or analog-digital converter.
FIG. 1 is a block diagram illustrating an internal configuration of a conventional audio amplifier. Referring to FIG. 1, a full digital amplifier 40 applied to a vehicle audio system generates a pulse width modulation (PWM) signal from a PCM signal input to a PWM generator 41, amplifies the PWM signal through a D-class amplifier 43 and then outputs the amplified PWM signal to an LC filter 50. The digital signal output from the full digital amplifier 40 has noise removed by passing through the LC filter 50 and is then output through a speaker 60 so that a user can listen to the signal.
In this state, the PWM generator 41 samples the input PCM signal at high speed and then generates a PWM signal through a noise shaping technique. The following processes are performed for the purpose of this signal processing.
First, a digitized sound source signal is input.
Second, the input signal is over-sampled eight times faster than the existing sampling frequency.
Third, noise is moved posterior to the audio frequency through the noise shaping technique.
Fourth and finally, the digital signal is amplified through the D-class amplifier, and a sound signal in an analog form is then extracted and output through the LC filter, thereby reproducing a sound source.
However, in the conventional audio amplifier to which the full digital amplifier is applied, there is a problem in that harmonic noise can be generated in the period of a sampling frequency when a digital signal is sampled at high frequency in signal processing. In this state, when two or three times harmonic noise is generated in the AM frequency region that is a region of radio broadcast frequencies, the harmonic noise has influence on the noise itself. In addition, the harmonic noise may have influence on the harmonic frequency for each carrier signal, even in the FM frequency region.